This invention relates generally to an electrophotographic printing machine, and more particularly concerns an improved method and apparatus for controlling the lateral movement of a moving belt.
In the process of electrophotographic printing, a photoreceptor belt is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the belt is exposed to a light image of an original document being reproduced. Exposure of the charged belt selectively discharges the charge thereon in the irradiated areas. This records an electrostatic latent image on the belt corresponding to the informational areas contained within the original document. After the electrostatic latent image is recorded on the belt, the latent image is developed by bringing a developer mixture into contact therewith. Generally, the developer mixture comprises toner particles adhering triboelectrically to the carrier granules. The toner particles are attracted from the carrier granules to the latent image forming a toner powder image on the photoreceptor belt. The toner powder image is then transferred from the belt to a copy sheet. Finally, the copy sheet is heated to permanently affix the toner particles thereto in image configuration. This general approach was originally disclosed by Carlson in U.S. Pat. No. 2,297,691 and has been further amplified and described by many related patents in the art.
Since the belt passes through many processing stations during the printing operation, lateral alignment thereof is critical and must be controlled within prescribed tolerances. As the belt passes through each of these processing stations, the location of the latent image must be precisely defined in order to optimize the operations relative to one another. If the position of the latent image deviates from processing station to processing station, copy quality may be significantly degraded. Hence, lateral movement of the photoreceptor belt must be minimized so that the belt moves in a predetermined path.
Similarly, document handling systems frequently employ belts to transport original documents to and from the exposure station. The lateral movement of belts used in document handling systems must also be controlled in order to insure the correct positioning of the original documents relative to the optical system of the exposure station.
There is a special need for precise control of a belt's lateral movement in a machine designed for multichromatic (color copy) image reproduction. In making multichromatic reproductions with an apparatus utilizing, for example, a moving charged photoreceptor belt, charge patterns corresponding to related color separation images may be formed in successive image frames of the belt. Such patterns are developed with pigmented marking particles to form transferable images. Each image is transferred sequentially to a respective receiver member whereby each image forms one of the several color separations for the multicolor reproduction. The sequential image transfer must be accomplished with high accuracy in order to obtain quality output of separations for faithful multicolor reproduction. In such color applications, transferable images generated from such successive "master" separations must be properly aligned for accurate superimposed registration during the creation of a multicolor composite print.
Therefore, during the production of such a separation, lateral movement of the belt during belt rotation must be closely controlled.
Ideally, if the photoreceptor belt was perfectly constructed and entrained about perfectly cylindrical rollers mounted and secured in an exactly parallel relationship with one another, the velocity vector of the belt would be substantially normal to the longitudinal axis of the roller and there would be no lateral walking of the belt. However, in actual practice, this is not feasible. Frequently, the velocity vector of the belt approaches the longitudinal axis or axis of rotation of the roller at an angle. This produces lateral movement of the belt relative to the roller. Alternatively, the axis of rotation of the roller may be tilted relative to the velocity vector of the belt. Under these circumstances, the belt will also move laterally. Thus, the belt must be tracked or controlled to regulate its lateral position.
Numerous methods of controlling the lateral movement of the photoreceptor belt maintain the belt within desired parameters have been proposed and implemented. Certain of these control schemes use a characteristic steering coefficient when determining proper correction procedures. Generally, the characteristic steering coefficient is calculated for a prototype machine and this calculated coefficient is then transferred or used in a fixed manner by later manufactured machines. However, each manufactured machine will be different from any other machine, therefore the calculated steering coefficient of the prototype will not be ideally suited for each machine. Additionally, over time and/or when machine is serviced, characteristics of the machine will change. Thus, the use of a fixed characteristic steering coefficient will increase inaccuracies in the belt control scheme.
Accordingly, it is an object of the present invention to improve the system controlling the lateral movement of the photoreceptor belt employed in an electrophotographic printing machine including a method for repeatedly updating the steering coefficient.